A standard motor-vehicle door latch system has at least one key-operated cylinder, a plurality of latches one of which is directly operated by the cylinder, and a central controller. In a sophisticated modern system the key is movable from a center position to one side to lock or unlock the door and to the other side to set the system in an antitheft mode. The controller normally unlocks, but does not unlatch, the other latches when the key-operated latch is unlocked. In the antitheft position the central controller blocks operation of all the latches so the respective doors cannot even be opened from inside the vehicle. The advantage of such a system is that the doors can be securely locked from one location and that, as in a standard power-lock arrangement, all the doors can similarly be unlocked from this one location.
In European patent 0,447,818 of K. Claar (based on a German priority of 20 Mar. 1990) a system is described which uses Hall-effect sensors to ascertain the key position, eliminating a direct mechanical link. Such an arrangement uses a sensor of the type described in German utility model 9,415,257 published 5 Jan. 1995 to detect the key position. Such a sensor incorporates circuitry so that when a magnetic-field of a strength exceeding a predetermined threshold passes through it, the sensor is tripped to generate an output. Unfortunately such an arrangement is susceptible of manipulation, for instance by a would-be thief who can hold a large permanent magnet against the vehicle door to actuate the device and open the vehicle.
Thus commonly owned patent application Ser. No. 08/902,469 filed 29 Jul. 1997 describes a motor-vehicle door-latch system that has a key cylinder pivotal about an axis from a center starting position into a pair of opposite end positions flanking the center position, a key insertable into the cylinder only in the starting position thereof, a latch operable by the cylinder on displacement of same into one of the end positions, and an arcuate magnet pivotal about the axis, polarized generally parallel to the axis, and coupled to the cylinder for joint angular movement therewith. A pair of Hall-effect sensors are juxtaposed with the magnet and positioned such that in the central position the magnet is closely juxtaposed with both sensors, in the one end position the magnet is closely juxtaposed with one of the sensors and is spaced from the other of the sensors, and in the other end position the magnet is closely juxtaposed with the other sensor and is spaced from the one sensor. A lock system connected to the sensors has an antitheft mode initiated by the sensors on displacement of the cylinder into the other of the end positions.
This system will respond very accurately to the position of the arcuate magnet, but will be difficult to fool. If a would-be thief simply applies a large magnet to the area of the sensors, it will not be possible to duplicate the condition of only one of the sensors being traversed by the magnetic field of the system's magnet. In fact according to another feature of the invention the lock system is set to respond, normally by generating an alarm and/or setting the antitheft position, when such tampering is detected, as when neither of the sensors is traversed by the magnet's field. Thus in an attempt to overcome the system by judicious application of a magnetic field, the would-be thief is far more likely to trip the alarm than to open the latches controlled by the lock system.
While this system is relatively effective, it is still possible for it to be circumvented by carefully positioning a magnet on the key-position sensor. Admittedly this is fairly difficult, but is possible for a skilled thief.